1. Field of the Invention
The present invention relates to a lithium battery, and more particularly, to an organic electrolytic solution having high reliability since a battery thickness is maintained within an allowed range, and a lithium battery employing the same.
2. Description of the Related Art
As portable electronic devices, such as video cameras, cellular phones, notebook computers, etc., become more lightweight and have increasingly high performance, more research into batteries used as power supplies for such portable devices must be conducted. In particular, chargeable lithium secondary batteries have 3 times the energy density per unit weight than conventional lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, etc., and can be rapidly charged, and thus are a favorite topic of research at present.
In a lithium ion battery, transition metal compounds such as LiNiO2, LiCoO2, LiMn2O4, LiFePO4, LiNixCo1-xO2 (x=1, 2), Ni1-x-yCoxMnyO2 (0≦x≦0.5, 0≦y≦0.5) or oxides thereof with lithium are used as a cathode active material and lithium metal, an alloy of lithium, a carbonaceous material, a graphite material, etc. are used as an anode active material.
Electrolytes are divided into liquid electrolytes and solid electrolytes. When a liquid electrolyte is used, many safety problems, such as risk of fire due to leakage of the electrolytic solution and breakage of the battery due to vaporization of the electrolytic solution arise. To solve these problems, a solid electrolyte has been proposed for use instead of the liquid electrolyte. The solid electrolyte does not leak and is easily processed, and thus many studies have been conducted thereon, such as, inter alia, a polymer solid electrolyte, which is actively being studied. Currently known polymer solid electrolytes are divided into complete solid electrolytes containing no organic electrolytic solution and gel-type electrolytes containing an organic electrolytic solution.
Since a lithium battery is generally driven at a high operating voltage, a conventional aqueous electrolytic solution cannot be used. This is because lithium contained in an anode and an aqueous solution vigorously react with each other. Thus, an organic electrolytic solution in which a lithium salt is dissolved in an organic solvent is used in the lithium battery. In this case, organic solvents having high ionic conductivity and dielectric constant and low viscosity may be used. Since it is difficult to obtain a single organic solvent satisfying all these requirements, a mixed solvent including an organic solvent with a high dielectric constant and an organic solvent with a low dielectric constant, a mixed solvent including an organic solvent with a high dielectric constant and an organic solvent with a low viscosity, etc. are used.
A lithium secondary battery forms a passivation layer such as a solid electrolyte interface (SEI) film on a negative electrode surface upon initial charging through a reaction of carbon in the anode with the electrolytic solution. The SEI film enables the battery to be stably charged and discharged without further decomposition of the electrolytic solution (J. Power Sources, 51 (1994), 79-104). Also, the SEI film acts as an ion tunnel through which only lithium ions pass and solvates lithium ions to prevent cointercalation of an organic solvent, which moves with the lithium ions, into a carbon anode, thereby preventing a breakdown of the anode structure.
However, upon initial charging, gas is generated inside the battery due to decomposition of a carbonate-based organic solvent when forming the SEI film, which results in an increase in battery thickness (J. Power Sources, 72 (1998), 66-70). When the lithium battery is stored at high temperatures after being charged, the passivation layer gradually breaks down due to electrochemical energy and thermal energy increasing with time, the anode surface is exposed, and the amount of gas generated increases. Due to the generation of gas, an internal pressure of the battery increases, which causes a deformation of a central portion on a side of the battery, such as swelling of a rectangular lithium polymer battery in a certain direction. The increase in the internal pressure of the battery results in a local difference in adherence between electrode plates, thereby reducing performance and safety of the battery and making mounting of a set of lithium secondary battery difficult.
To solve the above problems, U.S. Pat. No. 5,353,548 discloses a method of preventing decomposition of a solvent and swelling of a battery by injecting a vinylene carbonate-based additive into an electrolytic solution to form a coating on a negative electrode surface through a reductive decomposition reaction of the additive. Similarly, JP Patent Publication Nos. 2002-367673 and 2002-367674 disclose an electrolytic solution including dicarboxylic diester and an aromatic compound having a molecular weight of 500 or less and JP Patent Publication No. 2004-006188 discloses an organic electrolytic solution including dicarboxylic acid, for example, oxalic acid or malonic acid, for a lithium secondary battery using a negative electrode active material which can be alloyed with lithium.
However, according to a method of forming a proper coating on a negative electrode surface by adding a small amount of an organic or inorganic material, the properties of the SEI membrane formed on the negative electrode surface vary according to the type of solvent used as the electrolytic solution and the electrochemical properties of the additive. In the case of the above-described conventional additives, since the resulting SEI film is still unstable, ion mobility in electrons is poor and the generation of gas due to the decomposition of the electrolytic solution is not sufficiently prevented.